Related fields include thin-film microwave devices with superconducting components and reduction of defects in dielectrics.
At temperatures <100 mK, amorphous silicon (a-Si) is an insulating dielectric. Its low cost and ease of fabrication make it attractive as an interlayer dielectric (ILD) for superconducting interconnects and components for planar microwave devices. However, a-Si can exhibit unwanted absorption at microwave frequencies (e.g., 3-300 GHz) and far-infrared frequencies (300-1000 GHz). This absorption can arise due to irregular structure of the amorphous material, such as from a combination of electronic mid-gap states caused by structural defects, and possibly atomic tunneling states, generally known as two-level systems (TLS). A reduction in this absorption would benefit high-frequency classical devices by reducing signal attenuation, dispersion and jitter. A reduction in this absorption would benefit quantum devices, such as rapid single flux quantum (RFSQ) circuits and reciprocal quantum logic (RQL) by increasing coherence times for quantum state signals.
ILD layers are typically 300-1000 nm thick. At this thickness, many surface treatments are ineffective to remove defects from the bulk of the film. This is also an inconvenient thickness to form by the precisely controlled methods of atomic layer deposition (ALD); each ALD cycle creates a monolayer on the order of 0.1 nm thick, therefore a layer hundreds of nm thick would take too long to be cost-effective.
Hydrogen (H) passivation can reduce absorption from mid-gap states in the a-Si, but H, particularly when weakly bonded, can introduce more absorption due to formation of (TLS. TLS effects originate in electrons, atoms, and other material components that may randomly change quantum states in the presence of an oscillating electric or magnetic field such as the microwave-frequency signals transmitted in superconducting microwave devices.
One type of TLS in silicon-based interlayer dielectrics is a hydrogen atom, usually from a Si precursor ligand, trapped between two dangling bonds from adjacent Si atoms. Because the Si—H bond is weak, the H easily breaks away from one Si atom and bonds to the other, and can just as easily switch back again.
Calculations have predicted that there is an optimal H content in a-Si where loss at 3-1000 GHz is minimized, and where either adding or subtracting H will increase the loss. Therefore, a need exists for methods to optimize the H content in a-Si ILD films for superconducting devices to minimize loss at microwave and far-infrared frequencies.